This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. OBJECTIVES: The primary objective of this study is to describe the safety and tolerability of three doses of MEDI-560 at 105 TCID50 when administered to children 6 to 12 months of age who are PIV3 seronegative at baseline and to infants 1 to 3 months of age regardless of baseline serostatus. The secondary objectives of this study are: 1. To describe the profile and magnitude of MEDI-560 shedding after each dose 2. To evaluate the immune response generated by multiple doses of MEDI-560 3. To evaluate the genotypic and phenotypic stability of any recovered vaccine virus DESIGN: This is a randomized, double-blind, placebo-controlled, multi-dose Phase 1/2a multi-center study designed to evaluate the safety, tolerability, viral shedding, immunogenicity, and genotypic and phenotypic stability of MEDI-560 in infants 1 to 12 months of age. MEDI-560 will be administered at a dosage of 105 TCID50 to two cohorts of subjects in a step-wise fashion, first in 6 to 12 month-old PIV3 seronegative children and subsequently in 1 to 3 month-old infants not screened for baseline serostatus. Three doses of MEDI-560 at 105 TCID50 or placebo will be delivered on a 0, 2 month ([unreadable]8 days), 4 month ([unreadable]8 days) schedule to all subjects. The target sample size for this study is 190 subjects, with 30 subjects enrolled into Cohort 1 and 160 subjects enrolled into Cohort 2. All subjects will be followed for 180 days after the final dose of study vaccine for serious adverse events and significant new medical conditions. The total duration of study participation for each subject will be 180 days after the final dose of study vaccine. The scope of our request to the CRC is only to use -70 freezer space to store biologics.